1. Field
Compounds containing a 4-[(E)-alk-2-enoyloxy]benzoate group, liquid crystalline mixtures which contain such compounds, the use of such compounds and, respectively, mixtures in electro-optical indicating devices.
2. Background
Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells ("twisted nematic") having a twisted nematic structure, STN cells ("super twisted nematic"), SBE cells ("super birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials should have a good chemical and thermal stability and, moreover, should be stable towards electric fields and electromagnetic radiation. At usual operating temperatures they should have a suitable mesophase, for example a nematic, cholesteric or tilted smectic phase. Further, the liquid crystal materials should have a low viscosity and in the cells should give short response times, low threshold potentials (V.sub.10) and a high contrast.
Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfill different requirements depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible. In addition to the general interest in liquid crystal materials having a low dielectric anisotropy, there has, however, recently been an increased interest in materials having a very high dielectric anisotropy and at the same time a high clearing point, especially for TN cells having a low threshold potential, for example, in the case of TN applications for so-called outdoor uses.
Since liquid crystals are usually used as mixtures of several components in order to optimize the properties, it is important that the components have a good miscibility with one another. Components which severely lower the threshold potential in mixtures even in low concentrations usually have the disadvantage that they severely lower the clearing point and significantly increase the viscosity. The problem is therefore to look for compounds having a sufficiently great influence on the threshold potential (V.sub.10) such that they can be used in concentrations in which neither an undesired lowering of the clearing point nor an undesired high viscosity occurs.